Add upgrade status for new major module versions?

Several users seem to expect that Upgrade Status also provides information about contrib modules that are available in a new major version, i.e. Image 1.x -> Image 2.x.

Such upgrading notes are usually provided in the release node of the new major version, or on the project page of a module. However, that's not always easy to find and sometimes not even complete - so let's think about how update|upgrade status could provide upgrade information for contrib modules.